Expandable spinal implants have height raising capabilities that are utilized once the implant is initially positioned. Such height raising capability may be utilized within the spine anteriorly, posteriorly, or both and to various extents, respectively to raise the front or back of the implant. More particularly, such implants have upper and lower surfaces of upper and lower portions that in an insertion position are collapsed relative to one another and in a deployed position are spaced further away from one another than in the collapsed position.
Expandable implants offer the advantage of allowing for the placement of a potentially larger implant through a smaller opening in a patient's body. One of the first expandable spinal implants allowing for the growth of bone from vertebral body to vertebral body through the implant was invented by Michelson which is disclosed in U.S. Pat. No. 5,776,199, filed Jun. 28, 1988.
The prior art expandable interbody spinal implants preferably were inserted from an anterior approach to the spine, an approach posterior to the vertebral transverse processes, or to either side of the spinal midline in pairs. Such expandable implants are adapted to increase in height at their leading ends or at their trailing ends from a collapsed state to an expanded state for the purpose of increasing spinal lordosis at that interspace. During installation of expandable interbody spinal implants, it is desirable that the surgeon have the ability to precisely control the implant with the appropriate instruments and methods to load the implant with appropriate bone growth promoting material, to insert the implant into the implantation space, to deploy the implant to a final expanded state, and to further load the implant with bone growth material if so desired.
The technology of these prior art devices is such that the implant when expanded can be anchored into the adjacent vertebra similar to how drywall anchors used in building construction expand as a threaded fastener is screwed into the anchor. To avoid turning or rotation saw toothed edges are provided on the exterior surfaces which embed themselves into the bone.
Also known in the prior art are expandable interbody spinal implants that are circumferentially expandable at one of their leading or trailing ends to expand both the height and the width of the implant. Such implants have an expansion mechanism that is moved from the trailing end through the interior of the implant to reach the leading end to expand the implant. Any bone growth material present within the interior of the implant would be forced out of the interior of the implant by the expansion mechanism passing therethrough. Accordingly, such implants cannot be effectively preloaded with bone growth promoting material prior to expansion of the implant.
There exists a need for a circumferentially expanding implant that is stable when positioned and is substantially hollow and substantially devoid of any elaborate or substantial space occupying expansion mechanism to permit preloading of the implant with bone growth promoting material prior to expansion of the implant. The expansion mechanism should not interfere with the capacity to compressively load osteogenic material such as bone or any other suitable material through the length of the implant and to have it extrude from the implant. The extrusion of the osteogenic material from the implant provides an increased volume of osteogenic material over a greater surface area of the adjacent vertebral bodies adjacent the disc space to be fused and beyond the surface area of contact of the implant to the vertebral bodies themselves. Surrounding the implant itself with additional fusion promoting substances in contact with the adjacent vertebral bodies may enhance the fusion process.